1. Technical Field
The present invention relates to a telephone system, and more particularly to a data communication circuit of a telephone system having a data communication terminal for allowing a user to establish either voice communication or data communication when a portable communication device such as a notebook computer is connected to the data communication terminal of the telephone system.
2. Background Art
Generally, communication systems such as telephone systems and facsimile systems having telephone handsets today are capable of allowing subscribers to establish voice or telephone communication as well as data communication over a single telephone line. Such facsimile systems are disclosed, for example, in U.S. Pat. No. 5,018,189 for Communication Apparatus With Telephone And Data Communication Capability issued to Kurosawa et al., and U.S. Pat. No. 5,428,673 for Data Communication Apparatus Having The Function Of Automatically Switching Data Communication And Telephone Communication issued to Nakagawa in which internal switching devices are provided to automatically switch data communication and telephone communication. These conventional switching device may be constructed external to the facsimile systems and alternatively telephone systems such as disclosed in U.S. Pat. No. 4,660,218 for Apparatus For Switching Telephone Line A Response Device, A Data Terminal Or A Telephone issued to Hoshirnoto, U.S. Pat. No. 4,991,200 for Interface Device For The Intercommunication Of A Computer And A Fax Machine issued to Lin and U.S. Pat. No. 5,187,736 for Telephone Data Communication Switching System issued to Moriizumi so that different communication systems can effectively share the same telephone line.
Conventional telephone systems also include switching arrangements to accommodate both voice and data communication over a single telephone line. For example, in U.S. Pat. No. 4,367,374 issued to Serrano and U.S. Pat. No. 4,907,267 issued to Gutzmer, a manually operated switch is inserted between a telephone handset and base to allow both voice and data transmission. When the switching arrangement is used with a computer system or a facsimile machine, the user must manually take the telephone handset off-hook and place a call to a remote station. When the remote station answers the call, the user must then manually prompt the modem to go off-hook and begin communications with the remote station. While the switching arrangement may be made automatic as disclosed, for example, in U.S. Pat. No. 5,444,770 for Telephone Handset Interface For Automatic Switching Between Voice And Data Communications issued to Davis et al., the switching between voice and data communications during a conversation remains the same and the interface device connected between the telephone handset and base to selectively couple a modem to the telephone base for data communications remains complex and sensible only in offices and home.
As portable communication devices such as notebook computers become increasing common in today business environment however, there is a need to facilitate data communications over conventional telephone systems and particularly to those telephone systems that are accessible to the public so that the users of the notebook computers can simply plug their notebook computers into the telephone systems to establish data communications over a shared telephone line.